The invention relates to a display tube comprising an evacuated envelope having a substantially rectangular display window which on its inside comprises a display screen and in front of which on its outside, substantially parallel to the display window, a light-transmitting second window is provided and a light-transmitting cooling liquid flows through the space between the display window and the second window. Such a display tube is known from Netherlands Patent Application No. 8003360 laid open to public inspection. The display screen of such a display tube often comprises a phosphor layer on which a frame is written by means of an electron beam. As a result of the electron bombardment the temperature of the display screen rises so that the luminous efficiency of the display screen decreases. This effect is terminal "thermal quenching". This is the case in particular in display tubes for projection television in which the display screens are scanned by electron beams having large beam currents so as to obtain the required great brightnesses. In order to counteract the reduction of the luminous efficiency it is known from the Netherlands Patent Application No. 8003360 laid open to public inspection to cool the display window and the display screen connected thereto. This is done in the manner described in the opening paragraph. A disadvantage of this way of cooling, however, is that inhomogeneities of the refractive index occur in the cooling liquid which are expressed in refractive patterns in the displayed picture.